[PATCH] cgroups: Don't attach task to subsystem if migration failed

[Ben Blum]

If a task has exited to the point it has called cgroup_exit()
already, then we can't migrate it to another cgroup anymore.

This can happen when we are attaching a task to a new cgroup
between the call to ->can_attach_task() on subsystems and
the migration that is eventually tried in cgroup_task_migrate().

In this case cgroup_task_migrate() returns -ESRCH and we don't
want to attach the task to the subsystems because the attachment
to the new cgroup itself failed.

Fix this by only calling ->attach_task() on the subsystems if
the cgroup migration succeeded.

Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Ben Blum <bblum@xxxxxxxxxxxxxx>
Acked-by: Paul Menage <paul@xxxxxxxxxxxxxx>
Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
---
 kernel/cgroup.c |   15 +++++++++------
 1 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1d2b6ce..84bdace 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2135,14 +2135,17 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 		oldcgrp = task_cgroup_from_root(tsk, root);
 		if (cgrp == oldcgrp)
 			continue;
-		/* attach each task to each subsystem */
-		for_each_subsys(root, ss) {
-			if (ss->attach_task)
-				ss->attach_task(cgrp, tsk);
-		}
 		/* if the thread is PF_EXITING, it can just get skipped. */
 		retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, true);
-		BUG_ON(retval != 0 && retval != -ESRCH);
+		if (retval == 0) {
+			/* attach each task to each subsystem */
+			for_each_subsys(root, ss) {
+				if (ss->attach_task)
+					ss->attach_task(cgrp, tsk);
+			}
+		} else {
+			BUG_ON(retval != -ESRCH);
+		}
 	}
 	/* nothing is sensitive to fork() after this point. */
 
-- 
1.7.5.4


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